Fragum erugatum is a small species of cockle, a marine bivalve mollusc in the family Cardiidae.

[2] Right and left valve of the same specimen: Fragum erugatum is native to warm shallow waters in Western Australia.

[3] Microfragum erugatum lives buried just below the sandy seabed, extending its siphons to the surface to draw in water in order to breathe and feed.

These microalgae are photosynthetic and transfer simple organic compounds to their mollusc host which it uses to supplement the planktonic particles it filters from the water.

By contrast, in Fragum erugatum, the molluscs remain small but thrive, becoming very numerous, sometimes being found at densities of 4,000 per square metre (11 sq ft).

The larvae form part of the zooplankton and drift with the currents until they settle on the seabed to undergo metamorphosis into juveniles.